The present invention relates to a novel, highly scratch-resistant multicoat paint system comprising a multicoat clearcoat. The present invention further relates to a novel process for producing highly scratch-resistant multicoat paint systems which is based on the wet-on-wet technique. The present invention relates not least to the use of the novel, highly scratch-resistant multicoat paint systems particularly in automotive OEM finishing and in industrial coating.
The requirements imposed on the resistance of topcoats which are exposed to weathering, especially insolation and acid rain, and also, even more frequently, to mechanical stress, especially that produced by wash-brush cleaning installations, are becoming increasingly more stringent. This is so most particularly for vehicle finishes, which have to meet these requirements while also satisfying very high optical demands regarding gloss, surface smoothness, and color.
To date this problem has been solved by applying to the substrates a multicoat paint system which, in accordance with the heightened environmental compatibility requirements, is constructed substantially from aqueous coating materials. In the case of metal substrates, this multicoat paint system is conventionally produced from an aqueous electrocoat material, a water-based primer-surfacer, an aqueous basecoat material, and a solventborne clearcoat material. The electrocoat material and the water-based primer-surfacer are each baked following their application, and then constitute the primer. The aqueous basecoat material is applied atop said primer and the system is subjected to interim drying. Over this coat, which is not yet fully cured, the clearcoat material is applied, after which the two coats are cured together (wet-on-wet technique). Where the substrates used comprise plastics, water-based primers are conventionally employed in place of the electrocoat materials.
The wet-on-wet technique for producing multicoat topcoat systems is described, for example, in the patents U.S. Pat. No. 3,639,147, DE-A-33 33 072, DE-A-38 14 853, GB-A-2 012 191, U.S. Pat. No. 3,953,644, EP-A-0 260 447, DE-A-39 03 804, EP-A-0 320 552, DE-A-36 28 124, U.S. Pat. No. 4,719,132, EP-A-0 297 576, EP-A-0 069 936, EP-A-0 089 497, EP-A-0 195 931, EP-A-0 228 003, EP-A-0 038 127, and DE-A-28 18 100.
The optical or visual appearance and the weather stability are primarily the responsibility of the two topmost coats of the multicoat paint systems. Thus, the aqueous basecoat gives the multicoat paint system its color and/or optical effects such as metallic effects or interference effects, while the clearcoat provides not only scratch and etch resistance, i.e., resistance to damaging environmental substances, but also the appearance qualities, i.e., the gloss, brilliance, and evenness. In this system, aqueous basecoat material and clearcoat material must be very precisely matched to one another in order to produce a system having the desired advantageous profile of properties.
Aqueous basecoat materials and the corresponding paint systems which substantially meet these requirements are known from the patents EP-A-0 089 497, EP-A-0 256 540, EP-A-0 260 447, EP-A-0 297 576, WO 96/12747, EP-A-0 523 610, EP-A-0 228 003, EP-A-0 397 806, EP-A-0 574 417, EP-A-0 531 510, EP-A-0 581 211, EP-A-0 708 788, EP-A-0 593 454, DE-A-43 28 092, EP-A-0 299 148, EP-A-0 394 737, EP-A-0 590 484, EP-A-0 234 362, EP-A-0 234 361, EP-A-0 543 817, WO 95/14721, EP-A-0 521 928, EP-A-0 522 420, EP-A-0 522 419, EP-A-0 649 865, EP-A-0 536 712, EP-A-0 596 460, EP-A-0 596 461, EP-A-0 584 818, EP-A-0 669 356, EP-A-0 634 431, EP-A-0 678 536, EP-A-0 354 261, EP-A-0 424 705, WO 97/49745, WO 97/49747, and EP-A-0 401 565.
These aqueous basecoat materials principally comprise anionically stabilized polyurethanes, with polyfunctional amines and/or amino alcohols, such as triethylamine, methylethanolamine and/or dimethylethanolamine, being used as neutralizing agents.
The solventborne clearcoat materials are commonly one-component (1K), two-component (2K) or multi-component (3K, 4K) clearcoat materials such as are described, for example, in the patents EP-A-0 604 992, WO 94/22969, EP-A-0 596 460, and WO 92/22615.
Although this route leads to multicoat paint systems that satisfy the optical requirements, the one-component (1K) clearcoat often lack sufficient weathering stability, while the two-component (2K) or multi-component (3K, 4K) clearcoats are often not sufficiently abrasion-resistant.
Moreover, there has been no lack of attempts to master these problems through the use of powder clearcoat materials, such as are known, for example, from the German patent DE-A-42 22 194, powder slurry clearcoat materials, such as are known, for example, from the German patent DE-A-196 13 547, or UV-curable clearcoat materials, such as are described, for example, in the patents EP-A-0 540 884, EP-A-0 568 967, and U.S. Pat. No. 4,675,234. In certain circumstances, however, other disadvantages are then introduced, such as a not entirely satisfactory intercoat adhesion, without the problems of scratch resistance or of etch resistance being fully solved.
The patents U.S. Pat. No. 5,474,811, U.S. Pat. No. 5,356,669, U.S. Pat. No. 5,605,965, WO 94/10211, WO 94/10212, WO 94/10213, EP-A-0 594 068, EP-A-0 594 071, and EP-A-0 594 142 disclose coating materials which comprise at least one oligomer and/or polymer containing at least one pendant carbamate group of the formula
xe2x80x94Oxe2x80x94(CO)xe2x80x94NH2
and at least one crosslinking agent containing at least two functional groups which undergo crosslinking reactions with the carbamate group. These known coating materials produce clearcoats which possess extremely high scratch resistance and etch resistance. Cross-linking agents principally employed are highly etherified melamine-formaldehyde resins. The cross-linking itself is catalyzed by acid, catalysts used preferably comprising strong protic acids, especially sulfonic acids, which as a general rule are blocked with amines.
However, these advantageous coating materials cannot be used together with the abovementioned aqueous basecoat materials to construct multicoat paint systems, since if this is done there are frequently surface defects in the clearcoat owing to the shrinkage effect known as wrinkling.
It is an object of the present invention to provide a novel multicoat paint system which no longer has the disadvantages of the prior art but which instead possesses not only an outstanding profile of optical properties and an outstanding appearance but also high scratch resistance and etch resistance, and in which surface defects no longer occur. A further object of the present invention was to provide a novel process for producing multicoat paint systems which, simply, safely, and reliably, produces the novel multicoat paint systems.
The invention accordingly provides the novel, highly scratch-resistant multicoat paint system on a primed or unprimed substrate, comprising
(A) a basecoat,
(B) a first clearcoat as intermediate coat, and
(C) a second clearcoat as the topmost coat
lying above one another in that order, wherein the second clearcoat (C) is producible from a coating material (C) comprising
(C1) at least one oligomer and/or polymer containing at least one pendant carbamate group and/or allophanate group (c11), especially carbamate group (c11), of the formula:
xe2x80x94Oxe2x80x94(CO)xe2x80x94NH2xe2x80x94Oxe2x80x94(CO)xe2x80x94NHxe2x80x94(CO)xe2x80x94NH2
xe2x80x83and
(C2) at least one crosslinking agent containing at least two functional groups (c21) which undergo crosslinking reactions with the carbamate group and/or allophanate group (c11).
In the text below, the novel, highly scratch-resistant multicoat paint system is referred to as the xe2x80x9cmulticoat system of the inventionxe2x80x9d.
The invention also provides the novel process for producing a highly scratch-resistant multicoat paint system, comprising at least the following steps:
(I) applying a basecoat material (A) to a primed or unprimed substrate,
(II) carrying out interim drying of the applied base-coat material (A),
(III) applying a first clearcoat material (B), and
(IV) together curing the films (A) and (B) applied in steps (I) and (III) (wet-on-wet technique),
which comprises
(V) applying to the clearcoat (B) cured in step (IV) a materially different second clearcoat (C) comprising
(C1) at least one oligomer and/or polymer containing at least one pendant carbamate group and/or allophanate group (c11), especially carbamate group (c11), of the formulae:
xe2x80x94Oxe2x80x94(CO)xe2x80x94NH2xe2x80x94Oxe2x80x94(CO)xe2x80x94NHxe2x80x94(CO)xe2x80x94NH2
xe2x80x83and
(C2) at least one crosslinking agent containing at least two functional groups (c21) which undergo crosslinking reactions with the carbamate group and/or allophanate group;
and
(VI) carrying out curing.
In the text below, the novel process for producing a highly scratch-resistant multicoat paint system is referred to for the sake of brevity as the xe2x80x9cfirst process of the inventionxe2x80x9d.
The present invention further provides the novel process for producing a highly scratch-resistant multicoat paint system, comprising at least the following steps:
(I) applying a basecoat material (A) to a primed or unprimed substrate,
(II) carrying out interim drying of the applied basecoat (A), and
(III) applying a first clearcoat material (B),
which comprises
(IV) carrying out interim drying of the applied clearcoat material (B) and
(V) applying to the interim-dried clearcoat film (B) the materially different second clearcoat film (C), and then
(VI) baking the films (A), (B) and (C) together.
In the light of the prior art it was surprising and unforeseeable for the skilled worker that the object on which the present invention is based could be achieved by means of the multicoat system of the invention and the process of the invention. A particular surprise was that there were no problems of intercoat adhesion despite the multicoat construction of the clearcoat.
The key constituent of the multicoat system of the invention is the second clearcoat (C).
It is producible from a coating material (C) comprising as a first essential component at least one oligomer and/or polymer (C1) containing at least one pendant carbamate group and/or allophanate group (c11), especially carbamate group (c11), of the formulae:
xe2x80x94Oxe2x80x94(CO)xe2x80x94NH2xe2x80x94Oxe2x80x94(CO)xe2x80x94NHxe2x80x94(CO)xe2x80x94NH2.
Suitable oligomers and/or polymers (C1) include in principle all customary and known oligomers or polymers. Examples of suitable oligomers and polymers are linear and/or branched and/or block, comb and/or random addition copolymers of ethylenically unsaturated monomers, especially poly(meth)acrylates and also polyesters, alkyds, polyurethanes, acrylated polyurethanes, acrylated polyesters, polylactones, polycarbonates, polyethers, epoxy resin-amine adducts, (meth)acrylate-diols, partially saponified polyvinyl esters or polyureas, of which the copolymers of ethylenically unsaturated monomers, especially poly(meth)acrylates, are particularly advantageous and are therefore used with particular preference.
The carbamate group (c11) may be introduced by incorporating monomers containing these groups (c11). Examples of suitable monomers of this kind are ethylenically unsaturated monomers which contain a carbamate group or an allophanate group (c11).
Examples of suitable ethylenically unsaturated monomers containing a carbamate group (c11) are described in the patents U.S. Pat. No. 3,479,328, U.S. Pat. No. 3,674,838, U.S. Pat. No. 4,126,747, U.S. Pat. No. 4,279,833, and U.S. Pat. No. 4,340,497.
Alternatively, the carbamate group (c11) may be introduced into the oligomers and/or polymers (C1) by means of polymer-analogous reactions. Examples of suitable methods of this kind are known from the patents U.S. Pat. No. 4,758,632, U.S. Pat. No. 4,301,257, and U.S. Pat. No. 2,979,514.
Examples of poly(meth)acrylates (C1) whose use is particularly preferred in accordance with the invention are known from the patents U.S. Pat. No. 5,474,811, U.S. Pat. No. 5,356,669, U.S. Pat. No. 5,605,965, WO 94/10211, WO 94/10212, WO 94/10213, EP-A-0 594 068, EP-A-0 594 071, and EP-A-0 594 142.
In order to introduce allophanate groups (c11), oligomers and polymers having at least two, preferably at least three, primary and/or secondary, but especially primary, hydroxyl groups are transallophanatized with alkyl and aryl allophanates at from 30 to 200xc2x0 C., preferably from 50 to 160xc2x0 C., with particular preference from 60 to 150xc2x0 C., and in particular from 80 to 140xc2x0 C. The reaction is conducted in solution or in bulk, preferably in solution. It is advisable to add customary and known inhibitors to the reaction mixture, such as trialkyl phosphites, especially triisodecyl phosphite. It is further advantageous to use customary and known transesterification catalysts such as tin compounds, especially dibutyltin dioxide.
Examples of suitable allophanates for use in accordance with the invention are methyl, ethyl, propyl, butyl, pentyl, and phenyl allophanate, of which methyl and ethyl allophanate are particularly advantageous and are therefore used with particular preference in accordance with the invention.
Oligomers and polymers suitable for use in accordance with the invention and having at least two, preferably at least three, primary and/or secondary, but especially primary, hydroxyl groups include preferably linear and/or branched and/or block, comb and/or random addition copolymers of ethylenically unsaturated monomers, especially poly(meth)acrylates and also polyesters, alkyds, polyurethanes, acrylated polyurethanes, acrylated polyesters, polylactones, polycarbonates, polyethers, epoxy resin-amine adducts, (meth)acrylate-diols, partially saponified polyvinyl esters, polyureas, oligomeric polyols obtainable by hydroformylation and subsequent hydrogenation from oligomers themselves obtained by metathesis reactions of acyclic monoolefins and cyclic monoolefins; or aliphatic polyols, of which the copolymers of ethylenically unsaturated monomers, especially the poly(meth)acrylates, are particularly advantageous and are therefore used with particular preference.
Besides the hydroxyl groups, the abovementioned oligomers and polymers may also include functional groups such as acryloyl, ether, amide, imide, thio, carbonate or epoxide groups.
The second essential component of the coating material (B) for use in accordance with the invention is the crosslinking agent (C2).
Examples of suitable crosslinking agents (C2) are amino resins, especially melamine-formaldehyde resins, polysiloxanes, compounds containing at least one cyclic carbonate group, and/or polyanhydrides, but in particular amino resins.
In this context it is possible to use any amino resin suitable for topcoat materials or transparent clearcoat materials, or a mixture of such amino resins (C2). Particularly suitable are the customary and known amino resins some of whose methylol and/or methoxymethyl groups have been defunctionalized by means of carbamate or allophanate groups. Crosslinking agents of this kind are described in the patents U.S. Pat. No. 4,710,542 and EP-B-0 245 700 and also in the article by B. Singh and coworkers, xe2x80x9cCarbamylmethylated Melamines, Novel Crosslinkers for the Coatings Industryxe2x80x9d, in Advanced Organic Coatings Science and Technology Series, 1991, volume 13, pages 193 to 207.
Examples of suitable siloxanes (C2) are siloxanes containing at least one trialkoxy- or dialkoxysilane group such as trimethoxysiloxane.
An example of a suitable polyanhydride (C2) is polysuccinic anhydride.
As the further essential component the coating material (C) for use in accordance with the invention comprises at least one blocked or unblocked acid as crosslinking catalyst (C3).
Examples of suitable crosslinking catalysts (C3) are p-toluenesulfonic acid, dinonylnaphthalenedisulfonic acid, dodecylbenzenesulfonic acid, and acidic phosphates. Further examples of suitable crosslinking catalysts (C3) are disclosed in the patent U.S. Pat. No. 5,605,965. Examples of suitable blocking agents are amines.
The coating material (C) for use in accordance with the invention may comprise at least one compound (C4) containing at least one epoxide group. The compound in question is preferably an oligomer or polymer containing pendant epoxide groups. Examples of suitable oligomers or polymers come from the classes of polymer mentioned above. Particular preference is given to using poly(meth)acrylates containing pendant epoxide groups. The compounds (C4) may be employed advantageously in amounts of from 0.1 to 15, preferably from 0.5 to 10, with particular preference from 0.8 to 5, and in particular from 1 to 2% by weight, based on the solids content of the coating material (C).
The coating material (C) may further comprise customary and known solvents (C5) and/or additives (C6).
Examples of suitable solvents (C5) are esters such as butyl acetate or methyl octoate isomers (Exxate(copyright)) or aromatic solvent mixtures such as Solvesso(copyright).
Examples of suitable additives (C6) are UV absorbers; free-radical scavengers; Theological agents; silicas; reactive and inert nanoparticles such as are described, for example, in patent EP-A-0 832 947; colloidal metal hydroxides having blocked isocyanate groups, such as are described, for example, in the patent EP-A-0 872 500; slip additives; polymerization inhibitors; defoamers; leveling agents or film-forming auxiliaries such as cellulose derivatives. What is important is that these additives (C6) do not adversely effect, but instead advantageously vary and improve, the transparency and other particular advantageous properties of the clear-coat (C). Further examples of suitable additives (C6) are disclosed in the patent U.S. Pat. No. 5,605,695.
The composition of the coating materials (C) for use in accordance with the invention, i.e., the quantitative relationship of their components, may vary widely and is guided by the profile of properties of the clearcoat (C) and by its harmonization with the profile of properties of the clearcoat (B). Using his or her knowledge of the art, with or without the assistance of simple preliminary tests, and possibly using the prior art as a guideline, the skilled worker is able to determine suitable compositions. In general it is advisable to choose the proportions disclosed in the patents U.S. Pat. No. 5,474,811, U.S. Pat. No. 5,356,669, U.S. Pat. No. 5,605,965, WO 94/10211, WO 94/10212, WO 94/10213, EP-A-0 594 068, EP-A-0 594 071, and EP-A-0 594 142.
The preparation of the coating material (C) from its components has no special features but instead takes place in a customary and known manner by mixing of the constituents in suitable mixing units such as dissolvers.
The second essential constituent of the multicoat system of the invention is the first clearcoat (B).
Coating materials suitable for its production in accordance with the invention include all coating materials (B) which can be applied by the wet-on-wet technique to basecoats (A), especially aqueous basecoats (A).
The first essential component of the coating material (B) is at least one binder (B1).
Examples of suitable binders (B1) come from the classes of polymer mentioned above and are described, for example, in the patents EP-A-0 832 947, EP-A-0 604 992, WO 94/22969, EP-A-0 596 460, WO 92/22615, DE-A-42 22 194, DE-A-196 13 547, EP-A-0 540 884, EP-A-0 568 967, and U.S. Pat. No. 4,675,234.
Where the coating materials (B) are to be curable additionally or solely with actinic radiation, especially UV radiation and/or electron beams, they include binders (B1) which are curable with actinic radiation.
Suitable such binders (B1) include in principle all oligomeric and polymeric compounds curable with actinic radiation, especially UV radiation and/or electron beams, such as are commonly used in the field of UV-curable or electron-beam-curable coating materials. These radiation-curable coating materials normally include at least one, preferably two or more, radiation-curable binder(s) (B1), based in particular on ethylenically unsaturated prepolymers and/or ethylenically unsaturated oligomers.
Examples of suitable radiation-curable binders (B1) are (meth)acryloyl-functional (meth)acrylic copolymers, polyether acrylates, polyester acrylates, unsaturated polyesters, epoxy acrylates, urethane acrylates, amino acrylates, melamine acrylates, silicone acrylates, and the corresponding methacrylates. It is preferred to use binders (B1) that are free from aromatic structural units. Preference is therefore given to using urethane (meth)acrylates and/or polyester (meth)acrylates, with particular preference aliphatic urethane acrylates.
The binders (B1) contain functional groups (b11) which undergo crosslinking reactions with the functional groups (b21) of the crosslinking agents (B2) (principle of complementary groups).
Examples of suitable functional groups (b11) are thio, amino, hydroxyl, carboxyl, epoxy or (meth)acrylate groups.
The second essential component of the coating material (B) is at least one crosslinking agent. Said agent contains functional groups (b21) which undergo cross-linking reactions with the functional groups (b11) of the binders (B1) (principle of complementary groups).
Examples of suitable functional groups (b21) are anhydride, carboxyl, epoxy, isocyanate, blocked isocyanate, urethane, methylol, methylol ether, siloxane, amino, hydroxyl or beta-hydroxyalkylamide groups or ethylenically unsaturated groups crosslinkable with actinic radiation.
Examples of suitable complementary reactive functional groups (b11) and (b21) for use in accordance with the invention that undergo crosslinking reactions are compiled in the following overview. In the overview, the variable R stands for an acyclic or cyclic aliphatic radical, an aromatic radical and/or an aromatic-aliphatic (araliphatic) radical; the variables R1 and R2 stand for identical or different aliphatic radicals or are linked with one another to form an aliphatic or heteroaliphatic ring.
Examples of suitable crosslinking agents (B2) are the abovementioned crosslinking agents (C2). These cross-linking agents (B2) are employed principally in one-component (1K) clearcoats.
Further examples of suitable crosslinking agents (B2) for this end use are polyepoxides (b21), particularly all known aliphatic and/or cycloaliphatic and/or aromatic polyepoxides, based for example on bisphenol A or bisphenol F. Examples of suitable polyepoxides (B2) also include the polyepoxides obtainable commercially under the designations Epikote(copyright) from Shell, Denacol(copyright) from Nagase Chemicals Ltd., Japan, such as Denacol EX-411 (pentaerythritol polyglycidyl ether), Denacol EX-321 (trimethylolpropane polyglycidyl ether), Denacol EX-512 (polyglycerol polyglycidyl ether), and Denacol EX-521 (polyglycerol polyglycidyl ether).
Further examples of suitable crosslinking agents (B2) for this end use are tris(alkoxycarbonylamino)triazines of the formula: 
Examples of suitable tris (alkoxycarbonylamino) triazines (b21) are described in the patents U.S. Pat. No. 4,939,213, U.S. Pat. No. 5,084,541, and EP-A-0 624 577. In particular, the tris(methoxy-, tris(butoxy- and/or tris(2-ethylhexoxycarbonylamino)triazines are used.
The methyl butyl mixed esters, the butyl 2-ethylhexyl mixed esters, and the butyl esters are of advantage. They have the advantage over the straight methyl ester of better solubility in polymer melts, and also tend less toward crystallizing out.
Further examples of suitable crosslinking agents (B1) for this end use are the polyisocyanates (B2) described below whose free isocyanate groups are blocked with suitable blocking agents.
Examples of suitable blocking agents are the blocking agents known from the U.S. patent U.S. Pat. No. 4,444,954:
i) phenols such as phenol, cresol, xylenol, nitrophenol, chlorophenol, ethylphenol, t-butyl-phenol, hydroxybenzoic acid, esters of this acid, or 2,5-di-tert-butyl-4-hydroxytoluene;
ii) lactams, such as xcex5-caprolactam, xcex4-valerolactam, xcex3-butyrolactam or xcex2-propiolactam;
iii) active methylenic compounds, such as diethyl malonate, dimethyl malonate, ethyl or methyl acetoacetate, or acetylacetone;
iv) alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, n-amyl alcohol, t-amyl alcohol, lauryl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, methoxymethanol, glycolic acid, glycolic esters, lactic acid, lactic esters, methylolurea, methylolmelamine, diacetone alcohol, ethylenechlorohydrin, ethylenebromohydrin, 1,3-dichloro-2-propanol, 1,4-cyclohexyldimethanol or acetocyanohydrin;
v) mercaptans such as butyl mercaptan, hexyl mercaptan, t-butyl mercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol, methylthiophenol or ethylthiophenol;
vi) acid amides such as acetoanilide, acetoanisidinamide, acrylamide, methacrylamide, acetamide, stearamide or benzamide;
vii) imides such as succinimide, phthalimide or maleimide;
viii) amines such as diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine or butylphenylamine;
ix) imidazoles such as imidazole or 2-ethylimidazole;
x) ureas such as urea, thiourea, ethyleneurea, ethylenethiourea or 1,3-diphenylurea;
xi) carbamates such as phenyl N-phenylcarbamate or 2-oxazolidone;
xii) imines such as ethyleneimine;
xiii) oximes such as acetone oxime, formaldoxime, acetaldoxime, acetoxime, methyl ethyl ketoxime, diisobutyl ketoxime, diacetyl monoxime, benzophenone oxime or chlorohexanone oximes;
xiv) salts of sulfurous acid such as sodium bisulfite or potassium bisulfite;
xv) hydroxamic esters such as benzyl methacrylohydroxamate (BMH) or allyl methacrylohydroxamate; or
xvi) substituted pyrazoles, ketoximes, imidazoles or triazoles; and also
xvii) mixtures of these blocking agents, especially dimethylpyrazole and triazoles, malonates and acetoacetates, or dimethylpyrazol and succinamide.
Where the coating material (B) is designed as a two-component (2K) clearcoat material or multicomponent (3K, 4K) clearcoat material, crosslinking agents (B2) of higher reactivity, undergoing crosslinking reactions even at below 100xc2x0 C., preferably below 80xc2x0 C., may be used. Coating materials (B) of this kind will be chosen when the substrates cannot be subjected to high baking temperatures and/or when there is basically no need for high baking temperatures owing to the physical crosslinking of the basecoat material (A).
Examples of suitable reactive crosslinking agents (B2) are the polyepoxides described above.
Further examples of suitable reactive crosslinking agents (B2) are organic polyisocyanates, especially those known as paint polyisocyanates, containing free isocyanate groups attached to aliphatic, cycloaliphatic, araliphatic and/or aromatic moieties. It is preferred to use polyisocyanates (B2) containing from 2 to 5 isocyanate groups per molecule and having viscosities of from 100 to 10 000, preferably from 100 to 5 000 andxe2x80x94where manual mixing of the components is envisagedxe2x80x94in particular from 500 to 2000 mPas (at 23xc2x0 C.). If desired, small amounts of organic solvent, preferably from 1 to 25% by weight based on polyisocyanate alone, may be added to the polyisocyanates (B2) in order to improve the ease of incorporation of the polyisocyanate (B2) and, where appropriate, to reduce its viscosity to a level within the abovementioned ranges. Examples of suitable solvent additives for the polyisocyanates (B2) are ethoxyethyl propionate, amyl methyl ketone, and butyl acetate. Furthermore, the polyisocyanates (B2) may have been subjected to conventional hydrophilic or hydrophobic modification.
Examples of suitable polyisocyanates (B2) are described, for example, in Methoden der organischen Chemie, Houben-Weyl, volume 14/2, 4th edition, Georg Thieme Verlag, Stuttgart 1963, pages 61 to 70, and by W. Siefken, Liebigs Annalen der Chemie, volume 562, pages 75 to 136. Suitable examples include the isocyanato-containing polyurethane prepolymers (B2) which may be prepared by reacting polyols with an excess of polyisocyanates and which are preferably of low viscosity.
Further examples of suitable polyisocyanates (b21) are polyisocyanates having isocyanurate, biuret, allophanate, iminooxadiazinedione, urethane, urea and/or uretdione groups. Polyisocyanates (B2) containing urethane groups, for example, are prepared by reacting some of the isocyanate groups with polyols, such as trimethylolpropane and glycerol, for example. Preference is given to using aliphatic or cycloaliphatic polyisocyanates (b21), especially hexamethylene diisocyanate, dimerized and trimerized hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane 2,4xe2x80x2-diisocyanate, dicyclohexylmethane 4,4xe2x80x2-diisocyanate, diisocyanates derived from dimer fatty acids, as sold under the commercial designation DDI 1410 by Henkel and described in the patents WO 97/49745 and WO 97/49747, especially 2-heptyl-3,4-bis(9-isocyanatononyl)-1-pentylcyclohexane; or 1,2-, 1,4- or 1,3-bis(isocyanatomethyl)cyclohexane, 1,2-, 1,4- or 1,3-bis(2-isocyanatoeth-1-yl)cyclohexane, 1,3-bis(3-isocyanatoprop-1-yl)cyclohexane or 1,2-, 1,4- or 1,3-bis(4-isocyanatobut-1-yl)cyclohexane, 1,8-diiso-cyanato-4-isocyanatomethyloctane, 1,7-diisocyanato-4-iso-cyanatomethylheptane or 1-isocyanato-2-(3-isocyanato-propyl)cyclohexane, or mixtures of these poly-isocyanates (B2).
Very particular preference is given to using mixtures of polyisocyanates (B2) based on hexamethylene diisocyanate and containing uretdione and/or isocyanurate and/or allophanate groups, as are formed by catalytic oligomerization of hexamethylene diisocyanate using appropriate catalysts.
The coating material (B) may further comprise customary and known solvents (B3). Examples of suitable solvents (B3) are the above-described solvents (C5).
The coating material (B) may further comprise customary and known additives (B4) in effective amounts. Examples of suitable additives (B4) are the above-described additives (C6). Where the coating material (B) is curable with actinic radication, it may also include customary and known photoinitiators as additives (B4).
The coating material (B) may be in the form of a solventborne spray paint.
Alternatively, the coating material (B) may be a powder clearcoat material. For this purpose, components (B1) and (B2) and also, where appropriate, (B3) and (B4) are selected so that the resulting powder clearcoat material (B) is solid and forms a film before curing.
Alternatively, the coating material (B) may also be a powder slurry clearcoat material. For this purpose, the components are required to meet the conditions specified above. In addition, they must be selected so that the powder slurry particles can be dispersed stably in water.
The composition of the coating materials (B) for use in accordance with the invention, i.e., the quantitative relationship of their components, may vary widely and is guided by the profile of properties of the clearcoat (B) and by its harmonization with the profile of properties of the clearcoat (C). Using his or her knowledge of the art, with or without the assistance of simple preliminary tests, and possibly using the prior art mentioned initially as a guideline, the skilled worker is able to determine suitable compositions.
The preparation of the coating material (B) from its components has no special features but instead takes place in a customary and known manner by mixing of the constituents in appropriate mixing units. In the case of spray paints (B), dissolvers are examples of units used for this purpose; in the case of powder clearcoat materials or powder slurry clearcoat materials (B), extruders and mills are employed.
The third essential component of the multicoat system of the invention is the basecoat (A).
The basecoat (A) is producible from a coating material (A), preferably an aqueous coating material (A), in particular an aqueous basecoat material (A), and is the color- and/or effect-imparting constituent of the multicoat system of the invention.
The aqueous basecoat material (A) used with particular preference in accordance with the invention conventionally comprises the following key components in dispersion in water:
(A1) at least one anionically and/or nonionically stabilized polyurethane and
(A2) at least one color and/or effect pigment.
Examples of suitable color and/or effect pigments (A2) are metal flake pigments such as commercial aluminum bronzes, aluminum bronzes chromated in accordance with DE-A-36 36 183, and commercial stainless steel bronzes, and also nonmetallic effect pigments, such as pearlescent pigments and interference pigments, for example. Examples of suitable inorganic color pigments (A2) are titanium dioxide, iron oxides, Sicotrans yellow and carbon black. Examples of suitable organic color pigments (A2) are indanthrene blue, Cromophthal red, Irgazine orange and Heliogen green.
The coating material of the invention may further comprise organic and inorganic fillers (A3) in customary and known, effective amounts. Examples of suitable fillers are chalk, calcium sulfates, barium sulfate, silicates such as talc or kaolin, silicas, oxides such as aluminum hydroxide or magnesium hydroxide, or organic fillers such as textile fibers, cellulose fibers, polyethylene fibers or wood flour.
The coating material of the invention further comprises coatings additives (A4) in customary and known, effective amounts.
Examples of suitable additives (A4) are UV absorbers; free-radical scavengers; crosslinking catalysts, especially organometallic compounds, preferably organo-tin and/or organobismuth compounds or tertiary amines; reactive and inert nanoparticles such as are described, for example, in the patent EP-A-0 832 947, colloidal metal hydroxides containing blocked isocyanate groups, such as are described, for example, in the patent EP-A-0 872 500; slip additives; polymerization inhibitors; defoamers; adhesion promoters; leveling agents or film-forming auxiliaries, e.g. cellulose derivatives.
As additives (A4) it is possible in particular to use at least one rheology control additive. Examples of suitable rheology control additives (A4) are those known from the patents WO 94/22968, EP-A-0 276 501, EP-A-0 249 201, and WO 97/12945; crosslinked polymeric microparticles, such as are disclosed, for example, in EP-A-0 008 127; inorganic phyllosilicates such as aluminum magnesium silicates, sodium magnesium phyllosilicates and sodium magnesium fluorine lithium phyllosilicates of the montmorrilonite type; silicas such as Aerosils; or synthetic polymers containing ionic and/or associative groups, such as polyvinyl alcohol, poly(meth)acrylamide, poly(meth)acrylic acid, polyvinylpyrrolidone, styrene-maleic anhydride or ethylene-maleic anhydride copolymers and their derivatives or hydrophobically modified ethoxylated urethanes or polyacrylates. As rheology control additives (A4) it is referred to use polyurethanes and/or phyllosilicates.
If it is not to be cured by physical drying, the aqueous basecoat material (A) may further comprise at least one crosslinking agent (A5) in effective amounts.
Examples of suitable crosslinking agents (A5) are the above-described crosslinking agents (B2) suitable for use in one-component (1K) clearcoat materials. Where the aqueous basecoat material (A) is designed as a two-component (2K) or multicomponent (3K, 4K) clearcoat material, it comprises the above-described reactive crosslinking agents (B2).
Numerous examples of suitable anionically and/or non-ionically stabilized polyurethanes and also numerous examples of suitable compositions of aqueous basecoat materials (A) are disclosed in the patents EP-A-0 089 497 EP-A-0 256 540, EP-A-0 260 447, EP-A-0 297 576, WO 96/12747, EP-A-0 523 610, EP-A-0 228 003, EP-A-0 397 806, EP-A-0 547 417, EP-A-0 531 510, EP-A-0 581 211, EP-A-0 708 788, EP-A-0 593 454, DE-A-43 28 092, EP-A-0 299 148, EP-A-0 394 737, EP-A-0 590 484, EP-A-0 234 362, EP-A-0 234 361, EP-A-0 543 817, WO 95/14721, EP-A-0 521 928, EP-A-0 522 420, EP-A-0 522 419, EP-A-0 649 865, EP-A-0 536 712, EP-A-0 596 460, EP-A-0 596 461, EP-A-0 584 818, EP-A-0 669 356, EP-A-0 634 431, EP-A-0 678 536, EP-A-0 354 261, EP-A-0 424 705, WO 97/49745, WO 97/49747, and EP-A-0 401 565.
In terms of its method, the preparation of aqueous basecoat materials (A) has no special features but instead takes place in accordance with the customary and known methods by mixing of the components in appropriate mixing units such as dissolvers.
The multicoat system of the invention may be produced by any of a very wide variety of techniques. It is of advantage to produce it in accordance with the process of the invention.
The process of the invention starts from the substrate that is to be coated. The substrate may consist of metal, wood, plastic, glass or paper or may be a composite of these materials. The substrates in question preferably comprise automobile body parts, industrial components, or metal containers.
The substrate may have been provided with a primer coat. In the case of plastics, this coat is what is known as a water-based primer coat which is cured before the basecoat material (A), especially the aqueous basecoat material (A), is applied. In the case of metals, especially automobile body parts, the primer systems in question comprise customary and known, cured electrocoat materials which are overcoated with a primer-surfacer and baked.
In the procedure according to the invention, the basecoat material (A) is applied to the surface of the substrate in a step (I) and subjected to interim drying (step II), but without being cured.
In a step (III), a first clearcoat material (B) which subsequently forms the intermediate coat is applied to the surface of the basecoat film (A).
In step (IV), the films (A) and (B) are cured together (wet-on-wet technique). Depending on its composition, the film (B) may be cured thermally and/or with actinic radiation. As far as thermal curing is concerned, the baking temperature is guided in particular by whether the coating materials (A) and (B) are one-component (1K) or two-component (2K) or multicomponent (3K, 4K) systems. In the case of one-component (1K) systems, baking temperatures above 120xc2x0 C. are generally employed. In the case of two-component (2K) or multicomponent (3K, 4K) systems, the baking temperatures are normally below 100xc2x0 C., in particular below 80xc2x0 C.
In the invention, in step (V) the clearcoat film (C), materially different from the clearcoat film (B) is applied to the latter""s surface, and in step (VI) it is cured. In this case, baking temperatures of from 120 to 150xc2x0 C. are normally applied.
Alternatively, still in accordance with the invention, the second clearcoat film (C) may be applied wet-on-wet before being baked with the first clearcoat film (B) and the basecoat film (A).
In the context of the process of the invention, the coating materials (A), (B) and (C) may be applied to the substrates by means of customary application methods, such as spraying, knife-coating, brushing, flow coating, dipping or rolling, for example. Curing of the individual films may be carried out using the customary and known methods such as heating in a full scale oven, irradiation with IR lamps, and, where appropriate, with UV lamps as well.
Within the multicoat system of the invention, the thickness of the individual coats (A), (B) and (C) may vary widely. In accordance with the invention, however, it is of advantage if the basecoat (A) has a thickness of from 5 to 25 xcexcm, in particular from 7 to 15 xcexcm, and the two clearcoats (B) and (C) have a thickness overall of from 15 to 120 xcexcm, preferably from 40 to 80 xcexcm and in particular from 60 to 70 xcexcm. The ratio of the coat thicknesses may vary widely; on economic grounds, however, it is advantageous for the clearcoat (B) to be the thicker of the two coats.
The multicoat system of the invention has outstanding optical, mechanical and chemical properties. For instance, it is free of any surface defects such as shrinkage (wrinkling). It is of outstanding transparency and scratch resistance. Delamination owing to deficient intercoat adhesion is not observed. The etch resistance is outstanding. Accordingly, the coating material (C) for use in accordance with the invention is outstandingly suitable for enhancing all conventional clearcoat systems without any need for the customer to make substantial changes to the coating lines. This constitutes a further particular advantage of the multicoat systems of the invention.